Neil Basu said the main danger came from Islamic-State inspired extremists ‘in our midst’.

In an apparent reference to illegal Islamic schools, he said ‘unregulated’ education and home schooling were a breeding ground for home-grown terror. The counter-terror chief warned of the danger from extremists in ‘segregated, isolated’ communities.

He also highlighted weaknesses on our borders, calling for much stricter checks on arrivals. Warning that the terror threat would be severe for at least five more years, the Met deputy assistant commissioner said:

The security services were investigating 600 extremist plots;

Sixty probes have been opened in the past six weeks alone;

Border checks on roll-on, roll-off ferries had become a weakpoint for national security;

Mr Basu spoke out while briefing members of the Police Superintendents’ Association at their conference in Stratford-upon-Avon.

In a frank assessment of the terror threat, he said it would still be around when he retired in five years, adding: ‘It is not going to change. This was truly a summer like no other, it was truly a shift and not a spike, it is truly a new norm that we face.

‘The threat was returning fighters and now it is the threat in our midst. We stopped a lot of our jihadis travelling and some are not committed to the cause. But if they cannot travel then why not attack here?

‘Borders and ports are porous. There is a lack of biometrics and advanced passenger information. Our borders are not badly controlled but nevertheless they are still vulnerable.’

There have been repeated calls for the Government to tighten borders to protect against the threat from extremists.

The US routinely screens suspicious travellers and refuses entry to anyone deemed a potential risk. But Britain relies on intelligence provided by the same European agencies whose shortcomings were exposed by deadly attacks in major cities.

Hundreds of thousands of migrants have arrived in the European Union over the past year from the Middle East with few or no checks made on their identity.

Mr Basu said it was futile to try to profile exactly who the next attacker would be and where they would strike.

Metropolitan Police Service commissioner Cressida Dick (pictured) said 500 investigations into potential extremist plots were active in July

But he added: ‘Segregated, isolated communities, unregulated education and home schooling are a breeding ground for extremist and future terrorists.’ There are concerns that relatively unknown Islamic schools in the West Midlands and northern England could be harbouring extremists.

The Daily Mail has revealed that the authorities were powerless to force four heavily criticised Islamic schools to close.

They are suspected of hardline teaching, racism and holding banned extremist texts, but continue to operate as they fight the closure orders through the courts.

On the number of active investigations, Mr Basu said: ‘Every week we open more than we close.’ He said the total was now around 600. In late July, Cressida Dick, the new commissioner of the Metropolitan Police Service, warned that 500 investigations were active.

Mr Basu said arrests of suspected terrorists had soared by 75 per cent since March, when five people were murdered and more than 50 injured in Westminster.

He added that at its peak, calls to the anti-terror hotline rose to 3,000 a month – from an average of just a few hundred – leaving operators floundering.

It also emerged that the Government commissioned a review into protecting national landmarks, buildings and crowded public spaces with vehicle barriers.

In an apparent reference to illegal Islamic schools (file pic of the Islamic State of Iraq and the Levant flag), Mr Basu said ‘unregulated’ education was a breeding ground for terror in the UK

Senior officers believe low-tech attacks involving vehicles rented by jihadi knifemen, such as those who struck in London and Barcelona, are among the greatest threats. Mr Basu said that the cost of meeting the recommendations was ‘eye watering’ and ran to hundreds of millions of pounds.

He defended the Government’s controversial Prevent counter-extremism strategy, saying a small but vocal minority was responsible for most criticism.

The Government has ordered a root and branch review of all counter-terrorism work nationwide. Scores of extra armed officers, who work closely with special forces, have been made available to combat the threat of marauding gunmen. Police have warned it may be necessary to collect more data about suspects in order to spot extremists, a suggestion that raises privacy concerns.

Officers are also considering sharing previously secret information about terrorist suspects with other authorities.

As senior national coordinator for counter-terrorist policing, Mr Basu bears overall responsibility for manhunts for the most dangerous extremists.